Electrically powered vehicles including battery powered and hybrid vehicles have recently increased in popularity due to numerous factors including increases in costs in fuel as well as stricter regulations of vehicle emissions. Some electrical vehicles may utilize substantial amounts of electrical energy which may be dangerous to occupants of the vehicles. Accordingly, methods have been provided to determine an insulation resistance of the batteries of the vehicle with respect to the chassis.
One conventional method for measuring insulation resistance is defined by ECE 324 Addendum 99 regulation No 100, Annex 4. Such method specifies that the battery is to be fully charged for the test and the voltmeter utilized measures voltages in DC values and has an internal resistance of greater than 10 MOhms.
This method also provides that the measurement will be performed in two steps where the voltages from the positive terminal (V′1) and the negative terminal (V1) of the battery are each measured with respect to chassis ground of the vehicle in a first step. The method provides that the measurements are assumed to be absolute values of the measurement since V′1 is always positive with respect to the chassis and V1 is always negative with respect to the chassis.
In a second step, a resistance (Ro) of 500 Ohms/V is provided from the negative terminal to chassis ground if V1>V′1 and the voltage of the negative terminal is measured with respect to the chassis. Alternatively, the resistance is provided from the positive terminal to ground and the voltage is measured from the positive terminal to the chassis ground if V′1>V1.
The insulation resistance is calculated by Ri=(V1-V2)/V2×Ro if V1 is greater than V′1. Otherwise the insulation resistance is calculated by Ri=(V′1-V2)/V2×Ro.
Modeling of this method shows a 3.372% error when test insulation component resistances of 100 kOhms and 900 kOhms are respectively coupled from the chassis to the positive and negative terminals and used for a 600 V battery and R0 equal to the proper 500Ω/V of 300 kOhms. Furthermore, if RO is increased above 500 Ohms/V, the error of this method increases. For example, if an R0 of 480 kOhms is utilized the error is 4.929%. Finally, use of a voltmeter having less internal resistance than 10 MOhms can also significantly increase the error. Inherent inaccuracies in the method and sensitivity to impedance values stipulated in the method are believed to be sources of some of the error.
At least some aspects of the disclosure provide apparatus and methods for measuring insulation resistances with improved accuracy and with less constraints.